Simplify mesh by merging and filling holes
I have taken a look at your Blend file, and I first noticed that your mesh is quite messy. In these cases, using operations as you have mentioned (Merge, Fill, Decimate, Dissolve) will only worsen the problem.

Due to the messiness of the object in question, if possible, I would recommend rebuilding the mesh using clean, quad-based geometry.

First, in top view (Numkey 7), add a new plane over top your existing object (Shift + A > Mesh > Plane)

Next, move the plane along the X axis to the left edge of your mesh (G then X)



In edit mode (Tab), begin to move the vertices to the shape of the mesh. To select vertices, verify that you are in Vertex Select Mode (Ctrl + Tab > Vertex).

When you have shaped a section of the mesh, you can then extrude the edge of the plane to the right to add more detail. Use Ctrl + Tab > Edge to select whole edges then use E to extrude and repeat the previous vertex shuffling procedure.



Use Ctrl + R to add new loops to existing sections of  your mesh. With these methods you can add as much or as little detail to your new mesh as you like.

Here is a low-detail example I was able to match in under a minute: